Since the middle 1970's, catalytic converters have been required equipment for the treatment of exhaust gas from internal combustion engines in vehicles. The primary purpose of these devices is to convert, by catalytic means, the noxious exhaust gas components of hydrocarbon fueled engines into harmless materials, such as carbon dioxide, nitrogen and water. More recently, additional efforts have been made to trap and oxidize products of incomplete combustion, such as particulates of carbons, that are generated in relatively large quantities by diesel powered engines.
Up to the present, the supports for the catalysts or the particulate traps which promote the desired pollution reducing effects have been made from ceramic or metal monoliths in the form of a "honeycomb." Honeycomb type catalytic converters typically include a large number of narrow passageways through which the exhaust gas must traverse before exiting the catalyst assembly. The inner walls and/or passages of these monolithic honeycombs are coated with a precious metal catalyst, or a plurality of catalysts, such as platinum, palladium, rhodium and/or iridium, that promote the catalytic conversion of the noxious exhaust gas components. A typical cross section of a honeycomb catalyst includes, for example, 10 to 300 passages per square centimeter. Hot exhaust gas coming into contact with the catalyst material undergoes chemical changes which convert the noxious exhaust gas components into water, carbon dioxide and nitrogen.
Metal honeycomb catalytic converters are typically made from an arrangement of flexibile metal substrates or metal foils which provide a plurality of gas passages. "Metal foils," as that term is used in this specification, means any thin metal or metal alloy, having two main or major surfaces, that is used as a substrate for a coating layer. Typically, such metal foils are 0.001 to 0.010 inches thick and are used to carry various catalytic coatings, such as washcoats and catalyst materials. Preferably, such metal foils may have a thickness in the range of 0.0015 to 0.003 inches.
Dynamic changes to the catalytic converter may occur during use. For example, catalytic converters become heated to extremely high temperatures by virtue of the hot exhaust gases. The exhaust gas temperature of an internal combustion engine of an automobile can reach temperatures as high as 1000.degree. C. and higher, and likewise, the honeycomb element of a catalytic converter can reach these same high temperatures.
Metal foils have a positive thermal expansion coefficient and tend to expand or become somewhat larger in dimension as a result of exposure to high temperatures. When used as substrates in a honeycomb catalyst support, such a dimensional increase in the size of metallic foils can constrict the size of the exhaust gas flow passages in the honeycomb body, thereby causing an increase in the backpressure on the engine. In particular, if the metal foil substrate is attached to another part of the catalytic converter or contained within a housing, such as a jacket for the catalytic converter, such thermal expansion can cause pronounced backpressure problems.
In some cases, thermal expansion in metal foils is reversible by simply cooling the substrate. However, it has been found that metal foils typically used as supports in metal catalytic converters can become permanently enlarged after exposure to high temperatures during use of the catalyst. This permanent thermal expansion phenomenon is an irreversible increase in the dimensional size of a metal foil and will be hereinafter referred to as metal foil "growth" in this specification.
Metal foil growth have been associated with specific catalytic layers applied thereto. For example, thermal expansion in metal foils made from ferritic stainless steel has been reported to be greatest when the foils are coated with catalyst containing compounds of cerium, lanthanum and/or yttrium. K. Tanaka, et al., "Anomalous Expansion Of Stainless Steel Foil For Metal Honeycomb Catalytic Converters Induced By High-Temperature Oxidation," Tetsu to Hagane, Vol. 81. No. 8, pp. 79-84, 1995. This article is entirely incorporated herein by reference. Additionally, certain metal foil alloys have been found to be more susceptible to permanent thermal expansion than others. For example, stainless steel, such as FeCrAl alloys, typically are more susceptible to this permanent foil growth as compared to NiCr and NiCrAl alloys.
It is difficult to predict the degree to which metal foils will be permanently enlarged. For example, in laboratory testing, the amount of foil growth observed for an FeCrAl metal foil coated with a catalytic layer has been as high as 18% when the metal foil was exposed to 950.degree. C. for 64 hours. Typically, when a catalytic layer containing cerium and lanthanum was applied to a commercially available FeCrAl alloy metal foil, such as Allegheny Ludlum's Alfa IV, the average foil growth after exposure to 1000.degree. C. for 25 hours was 6.8%. Foil growths as high as 10% and as low as 3.7% were observed under these heating conditions.
Because of the difficulty in predicting the degree of permanent expansion of metal foils, it can be difficult to design a catalytic converter system that maintains the dimensional balance necessary to achieve adequate contact between the exhaust gas and the catalyst without producing excessive fuel consumption or backpressure on the engine. Additionally, changes in the dimensional size of the metal foils can impair the integrity of mechanical structures or joints in the converter, e.g., the changed size of the foils may cause welded or brazed joints to become weakened or detached, or it may cause the foil to escape from clamping devices, or the like. Furthermore, even after permanent thermal expansion has occurred in the metal foils, the foils are still subject to further thermal expansion during heating (i.e., the normal, reversible thermal expansion exhibited by metal parts). This additional expansion causes further stresses on joints and the mechanical integrity of the catalyst system.
Accordingly, there is a need in the art for novel processes and compositions to prevent and/or control the degree of permanent thermal expansion of a metal foil when exposed to high temperatures. There is also a need for metal catalyst supports and catalyst assemblies in which problems associated with metal foil growth are minimized during use in high temperature environments.